Rename "/" to "File System" in places view and in places sidebar

Rename "/" item inside places view and places sidebar from "Computer" to "File System".

Reasons:

• GTK applications can be ran on devices other than computer (from user perspective) like tablet or (in the future?) smartphone. "File System" is more neutral.
• GTK uses "File System" label for "/" path in file chooser button list. Make it consistent.
• It seems to me that "File System" is technically more correct than "Computer".
• On places view "Computer" item is placed in "On My Computer" section. So, GTK says that my computer is on my computer. Hmm, interesting.
• A lot of applications calls "/" path as "File System" (gedit as example). Be consistent with rest of the world.
• "File System" is hard to understand for casual computer user but casual user should not go to "/" at all. From the other hand, "Computer" is not obvious for advanced users as "File System".
• GVFS has file system called "Computer" which is completely different than "/". Nautilus users can go there by typing computer://. Nemo (cinnamon) and Caja (mate) users can go there by using button inside its toolbar. Different usage of the same term seems to be confusing for users. (Edit: also gnome-panel shows "computer://" in its menu bar.)

This MR affects file chooser dialog, Nautilus and File Roller.